This invention relates to a system for irradiating laser beams excited by solar energy on water to decompose water and producing methane or ethanol from the hydrogen thus obtained.
As a method for industrially producing hydrogen, the most ordinary method is by electrolyzing water. But if fossil energy is used to generate electric power used for electrolysis, it creates a burden on the environment. In order to use hydrogen as completely clean energy, it is necessary to inexpensively and efficiently produce hydrogen from water in a large amount without using fossil fuel. From this viewpoint, the importance of producing hydrogen by electrolyzing water using the solar energy will increase in the future.
An example of the method for using hydrogen as energy is the use of methane and methanol. Methanol is a kind of alcohols obtained by synthesizing hydrogen with carbon monoxide (CO) or carbon dioxide (CO2), and can be used to store hydrogen in a liquid state in a large amount. Methanol can be used as a fuel for fuel cells. If automotive engines are replaced with electric motors, there is a possibility that fuel cells may spread as the power source. By continuously supplying methanol together with an oxidizer such as air or oxygen, electric energy can be directly taken out. In view of such form of using hydrogen, there may arise a necessity for a system in which hydrogen obtained by electrolyzing water using the solar energy is converted to methanol and methanol is stored as fuel. Also, since methane is gaseous and can be more easily liquefied than hydrogen, it can be used as a substitute for natural gas.
An ordinary method of producing hydrogen using solar energy is to electrolyze water using electric energy obtained by solar battery generation. Another method is by optoelectrochemical reaction using semiconductor electrodes such as titanium oxide (TiO2) (called titanium oxide (IV) or titanium dioxide to be precise). Hereinbelow called titanium oxide). It is known as the Honda/Fujishima effect.
In a method for producing hydrogen using such semiconductor electrodes, a semiconductor electrode of titanium oxide as an anode and a platinum (Pt) electrode as a cathode are oppositely arranged in two chambers filled with water and insulated from each other. When the sunlight is irradiated on the semiconductor electrode through a transmission window, a reaction occurs in which oxygen is produced, and hydrogen is produced at the platinum electrode, so that electric power is obtained through a load.
As semiconductor electrodes, various kinds are being studied besides TiO2. As an example, JP patent publication 11-246985 proposes the use of zirconium dioxide (ZrO2), tantalum oxide (TaO5), niobium oxide (NbO5), phosphorus indium (InP) and Pd alloys.
On the other hand, methanol (CH3OH) is synthesized by reacting CO or CO2 contained in e.g. generating exhaust gas with hydrogen industrially produced separately under high pressure.CO+2H2→CH3OHCO2+3H2→CH3OH+H2O
Methane can also be obtained under high pressure according to the following reaction formulas:CO+3H2→CH4+H2OCO2+4H2→CH4+2H2O
Further, it is also well-known that the methanol obtained in the above manner can be converted to gasoline at a high temperature of about 400° C. by use of synthesized zeolite ZSM-5 as a catalyst.
But with a method of producing hydrogen by the electric power of solar batteries, the energy efficiency is as low as about 20%. Also, on the ground where there are variations in diurnal difference, weather and season, only about 15% of the sunlight power at its peak can be utilized. Further, solar batteries themselves are expensive. In a method of producing hydrogen using a semiconductor electrode of e.g. titanium oxide, a usable wavelength range is theoretically determined by the material of the semiconductor electrode. Thus it is impossible to use the optical energy of the sunlight over its entire wavelength. Thus, in any of these methods, the energy conversion efficiency in decomposition of water by the sunlight is as low as about several percents. Therefore, they cannot be used as a method of industrially producing hydrogen. Thus it is also impossible to produce methane or methanol by use of hydrogen manufactured using the solar energy at a lower cost than it is now.
Also, methane and methanol can be produced from methane hydrate as one of ordinary methods. But, since carbon dioxide is generated during the combustion step, the content of carbon dioxide gas in the atmosphere increases. Thus, the same result comes as using fossil fuel. But if a method of converting the solar energy to laser beams in outer space is established, it is possible to markedly improve the energy conversion efficiency. It is expected that industrial use of hydrogen and methanol becomes possible. In this method, the content of carbon dioxide gas does not increase because carbon dioxide gas in the air is used in a cyclic manner.
An object of this invention is to provide a methane/methanol producing system in which hydrogen is produced at a low cost by utilizing the solar energy and methane or methanol is produced using the hydrogen produced.